The present invention relates to an optical pickup and an optical disc unit which reproduce data recorded on an optical disc.
As to the background of the invention, for example, JP-A-10-269587 discloses an optical disc unit which is small-sized and thin, which can easily prevent occurrence of stray light and which can detect signals in both push-pull process and three-beam process at the same time.
Further, as to another background of the invention, for example, JP-A-2006-54006 discloses a method which can prevent an S/N ratio of a reproduced signal from greatly lowering upon removal of an offset from a tracking error signal by a push-pull process.
Among optical discs, there is presented a two-layer disc having signal recording surfaces stacked in two layers. For example, as to DVDs, a DVD-R and a DVD-RW are two-layer discs which can have a recording capacity which is about 2 times as high as that of a single layer optical disc. Further, there is presented the so-called Blu-ray Disc (which will be hereinbelow referred to as “BD”) which is used in a high density recording optical disc system and which is also a two-layer disc.
An optical pickup incorporated in an optical disc unit utilizes a reflected light beam from an optical disc, as a servo control signal in both focusing and tracking directions of an objective lens. Thus, should undesirable stray light be mingled in the reflected light beam to be used as a signal, a failure would be caused during detection of a signal.
An optical pickup which split an optical beam emitted from a laser source into three optical beams, that is, a 0-order light beam and ±1-order light beams, which then irradiates these light beams onto an optical disc, and which receives from reflected light beams by a detector, has raised such a problem that, in the case of reproduction of a two-layer disc, an unnecessary reflected light beam from one of layers would become a stray light component which becomes a disturbance component of a tracking error signal obtained from the other one of layers.
However, in the above-mentioned JP-A-10-269587, a diffraction zone is split for exclusion of the stray light component, so as to receive only a +1- or −1-order light beam. Thus, a diffracted beam has a light quantity which is not greater than a half of that of an original light beam, that is, a detected signal becomes less. Further, since ±1-order light beams are produced from different split zones, the rate among the optical quantities of light beams which are diffracted at the split diffraction zones are likely to be uneven, the positions of optical spots of the ±1-order light beams can be hardly located in point symmetry with respect to a 0-order light beam, thereby it is difficult to obtain a satisfactory servo signal.
Further, the JP-A-2006-54006 proposes a tracking error signal detection system capable of restraining lowering of an S/N ratio of a reproduced signal. This document does not concern a problem of stray light caused by an optical disc having a plurality of recording layers, such as a two layer disc, that is, there is disclosed no measures for solving such a problem that the stray light becomes a component disturbing a tracking error signal.